There are instances in which, during the operation of rotating machinery, the magnitude of the resultant force acting on a shaft may vary significantly, and its direction of application projected in the axial direction of the shaft may reverse. As a result, two opposed thrust bearings are typically used to restrain the axial motion of the shaft. One of the thrust bearings (i.e., the “active” thrust bearing) supports the axial loading of the shaft during nominal continuous operation of the rotating machinery. The function of the other thrust bearing (i.e., the “inactive” thrust bearing) is limited to supporting transient axial load on the shaft opposite in direction to the nominal continuous operating axial load. Transient axial or thrust loading on the shaft may cause the shaft to vibrate excessively, which may ultimately trigger the automatic shutdown of the rotating machinery.
Prior art methods of preventing or containing the development of such excessive vibration in rotating machinery due to transient axial movements of the shaft include supplying a flow rate of lubricant that is in excess of the amount needed to satisfy the hydrodynamic requirement of each thrust bearing in an attempt to completely fill the axial clearance gaps between the interfacing features of the shaft and the corresponding load-carrying surfaces of active and inactive thrust bearings. These prior art methods, however, contribute to increasing the parasitic power loss in the rotating machinery due to additional lubricant shearing, churning, and pumping losses.